1. Field of the Invention
The invention relates to an electromagnetically compensating force-measuring or weighing apparatus with a compensation coil that is wound upon a coil form and movable in the air gap of a stationary permanent magnet system, with a capacitive position sensor for the position of the compensation coil, and with an automatic volume control amplifier series-connected to the position sensor so as to follow it, and whose output current feeds the compensation coil and strives to hold it in a prespecified zero position.
2. Prior Art
Electromagnetically compensating force-measuring or weighing devices are known. An embodiment with capacitive position sensor is described, e.g., in U.S. Pat. No. 2,680,012. A general view relating to the various structures of capacitive position sensors is presented, e.g., by Christof Rohrbach in the "Handbuch des elektrischen Messens mechanischer Grossen" ("Handbook on Electric Measurement of Mechanical Values") (1967). In most cases, the capacity variation of two electrodes upon variation of the space therebetween is utilized. In high precision force-measuring or weighing devices, usually two capacities are provided which vary in opposite directions, in order to achieve a higher zero-point stability. One electrode is, in this case, mounted on the movable part of the force-measuring or weighing apparatus and can move between two stationary electrodes. In this process, the capacity of the movable electrode with relation to one of the stationary electrodes increases, while the capacity of the movable electrode with relation to the other stationary electrode decreases. At smaller demands made on the position sensor, two electrodes whose capacity in the zero position is of a prespecified rated value are sufficient.
A disadvantage of this structure according to the prior art is, on the one hand, the large space required for the aereal electrodes, and on the other hand, the electrodes may, e.g., be distorted by temperature gradients, so that another zero position of the compensation coil of the force measuring or weighing apparatus results, with a consequent indication error. Besides, the electrical screening of the electrodes is frequently difficult (see e.g., Th. Gast/W. Kastel in VDI Berichte (VDI Reports) 312, 1978, page 15).